This invention relates to a serrated sawing wire intended to cut hard materials. It also relates to a method for manufacturing such a wire and equipment for implementing the method of manufacture. Finally, it relates to a method of sawing using such a sawing wire.
In the description hereinbelow, the references between brackets ([ ]) refer to the list of references shown at the end of the text.
The technique of cutting hard materials with a wire is already used for the flow of blocks of monocrystalline or polycrystalline silicon, of semi-conductors, of crystals of magnetic materials, of quartz ceramics or of other brittle materials.
In the case of the production of wafers, i.e. slices of silicon, for the manufacture of photovoltaic solar cells, this technique of sawing by wire is used. The method begins with blocks of monocrystalline or polycrystalline silicon with a square or round section which are cut into thin slices with a thickness between 180 and 280 μm. A simple wire, typically with a diameter of 120 μm and a length from 600 to 900 km, comes from a spool and runs over a bench of four rollers mounted rotatingly around axes parallel to each other and distributed at the tops of a rectangle. The wire surrounds multiple times the rollers and as such forms layers of strands parallel to each other and regularly spaced. Two of the layers are horizontal and receive during the operations of cutting a block of silicon that rests on a layer by its own weight or is pushed by mechanical means. The wire is received at its exit by a receiving spool.
Each strand receives via a nozzle an abrasive paste that it drives to the block. Particles of abrasive 3 are interposed between the wire 1 and the block 2, as shown in FIGS. 12 and 13, which carries out a cutting process via erosion. Grooves 4 with increasing depth parallel to each other are as such carried out through the block 2, until the grooves exit and entirely pass through the block. The abrasive is in general silicon carbide (SiC) with particles 3 with a diameter between 10 and 15 pm. The abrasive paste is formed by a mixture of these particles and an oil or glycol.
This method requires a wire of high quality, with a high tensile strength. A rupture of the wire during the cutting process makes the block unusable.
The sawing wire is in general a single smooth and straight steel filament. It is obtained through wiredrawing on a wet wiredrawing machine and is wound on a spool.
The effectiveness of the sawing operation depends on many parameters such as the features of the material to be cut, the speed of the wire, the nature of the abrasive and of the abrasive paste and the properties of the wire. A determining parameter is the capacity of the wire to drive the abrasive paste and to remove the mixture of abrasive paste and particles drawn from the material, in such a way as to renew the abrasive on the work surface and to prevent blocking of the wire in the groove.
The driving capacities of a smooth wire are limited. It is observed that the particles of abrasive tend to roll between the wire and the surface of the material to be cut, as symbolised by the arrows in FIG. 13. The abrasive therefore damages the wire at the same time as it abrades the material, which results in limiting the length over which it can be used before risking its rupture. The speed of the abrasive is on average half that of the wire.
Another phenomenon is also observed during the beginning of the cut, shown in FIG. 16. When the wire comes into contact with the surface to be sawed, it is simply taut without being guided laterally. The wire rolls on the abrasive and oscillates around its position of equilibrium, in such a way that an initial furrow is dug with a width greater than that of the groove that is then carried out. This results in input fillets 40 of the grooves. This phenomenon results in a substantial increase in the total variation of the thickness for the wafers, which is a substantial loss of quality.
Document WO 90/12670 A1 [1] proposes a monofilament sawing wire comprising a textured outer surface, in such a way that the abrasive is better driven and is detached less easily from the wire. In the forms proposed, the surface of the wire is provided with microcavities or with circumferential grooves. However, the methods of manufacturing such a wire are slow and lack productivity. The cost price for the wire is as such very high. In addition, the wire is weakened by the texture which creates concentrations of stresses and chafing.
Documents EP 1 827 745 A1 [2] and JP 12-89527 [3] show a wire to which crimps have been conferred, i.e. zigzag shaped through a passage between pairs of embossing rollers. Although these solutions have a better driving of the abrasive, the abrupt changes in the curvature of the wire are weak points for the wire which limit the tension that the wire can withstand. In addition, the crimps reduce the longitudinal rigidity of the wire, which can cause blockage of the wire during the sawing process. In order to limit these risks, the speed of the wire is reduced during the sawing, which limits the productivity of the sawing method.
Document JP 2004-276207 A [4] also proposes a wire for sawing. In an embodiment, the wire comprises spiral serrations in order to reserve space between the wire and the groove in order to store and remove the material detached from the part to be cut. However, this wire is excessively flexible longitudinally and has the risk of looping before it enters into the groove.
Moreover, recent developments have resulted in a wire covered with abrasive, in such a way that it is not necessary to use the abrasive paste. The abrasive can be formed from various oxides, carbides, or diamond particles. The particles are maintained on the wire by a binder layer. However, such a wire remains difficult and expensive to produce.
The invention aims to provide a sawing wire that allows for a good driving of the abrasive, a high running speed, that is resistant while still being inexpensive to produce. It also aims to provide a method for manufacturing such a sawing wire and equipment that implements this method.